A thermal asperity (TA) is an unwanted event sometimes occurring within hard disk drives employing a magneto-resistive (MR) read head element of a flying head or slider assembly. One way to increase data storage densities in magnetic hard disk drives having disks of a given size is to "fly" the head structure closer and closer to the disk surface than heretofore. Flying heights of one micro-inch are now being approached in hard disk drives. Unfortunately, disk surfaces are not perfectly smooth when viewed in a micro-inch domain. Also, occasional particulate contaminants may come between the slider and the disk surface.
When an MR read element comes into contact with a rough spot of the disk, or collides with a minute, freely moving contaminating particle, virtually instantaneous heating of the MR element occurs. In response to this sudden heating, resistance of the thin film MR stripe element quickly increases. Since a constant bias voltage or current is applied to bias the MR element during reading operations, the sudden increase in resistance is sensed by a preamplifier as a dramatic, rapid change in bias voltage, or a large baseline signal shift. This unwanted electrical signal shift (referred to hereinafter as a "thermal asperity" or "TA") can be several times larger than a signal shift induced by magneto-resistance effect in response to a magnetic flux transition recorded on the disk as user or servo data, etc. Since the read channel has a dynamic operating range optimized to expected signal magnitudes attributable to flux transitions, the out-of-range TA signal shift causes the electronics of the read channel to saturate. Once the read channel saturates, a relatively long time is needed to bring the channel back to nominal baseline operating conditions, and the saturation condition may remain over several subsequent data bit cells. Under a saturation condition, the read channel electronics of the disk drive cannot detect any recorded data transitions accurately. This saturation condition causes data errors at the proximity of the thermal asperity.
Prior approaches directed to correcting these data errors employ conventional error-correction-codes, e.g. ECC, that operate to locate and correct the data errors. However, ECC is limited to correcting a predetermined maximum number of errors that may occur in a data stream.
If the precise location of the errors can be determined prior to operation of the ECC algorithm, then the prior maximum number of correctable errors in a data stream may be substantially increased. Presently, techniques for determining the location of these errors includes thermal asperity detectors or general error detectors that are included within read channel electronics. One problem with these techniques is that error locations cannot be determined with precision, rather, only approximate error locations can be determined.
Another problem with the prior techniques for determining the approximate location of these data errors is that actual errors may not be detected at all and thus their location never realized. In some instances, errors generated as a result of a thermal asperity may not have a voltage amplitude with sufficient amplitude to be detected by a thermal asperity detector. Nevertheless, this low amplitude error may be long enough in time to cause a plurality of data errors that cannot be located and thus are not correctable.
Since prior error location methods can only provide the proximity of a data error, prior ECC schemes need to begin operation substantially before and conclude substantially after the proximate error location to provide a buffer zone for uncertainty of a data error's location. This additional zone for which the ECC must attempt correction of data errors, will sometimes cause the length of the total region to be corrected, to be longer than the maximum correction span of the ECC, causing the error to be uncorrectable.
Thus, a hitherto unsolved need has remained for a more effective method for recovering data from a magnetic recording disk with an MR head in the presence of thermal asperities.